4,5-Dichloro-1,2-dithiol-3-one and .beta.-bromo-.beta.-nitrostyrene (hereinafter abbreviated as bromonitrostyrene) to be used in accordance with the invention are known to be biocides for industrial use. Japanese Patent Publication No. 14294/1977 discloses that 4,5-dichloro-1,2-dithiol-3-one is a compound effective in preventing microorganism-caused slime formation in industrial water systems, in particular in white water in paper mills and in cooling water. Japanese Laid-open Patent Application KOKAI No. 7701/1981, Japanese Patent Publication No. 42603/1982 and Japanese Laid-open Patent Application KOKAI No. 91108/1986 mention that the combined use of 4,5-dichloro-1,2-dithiol-3-one and a bromine-containing microbicidal/microbistatic agent can produce a synergistic antimicrobial effect. Thus, it is known that the combined use of 4,5-dichloro-1,2-dithiol-3-one on one hand and a bromoacetate ester (Japanese Laid-open Patent Application KOKAI No. 7701-1971), 2,2-bromonitro-1,3-propanediol (Japanese Patent Publication No. 42603/1982) or dibromonitrilopropionamide (Japanese Laid-open Patent Application KOKAI No. 91108/1986) on the other can exhibit a several times enhanced antimicrobial activity, namely a marked synergism.
4,5-Dichloro-1,2-dithiol-3-one is in practical use as a slimicide in papermaking white water systems and industrial cooling water systems, either alone or in combination with a bromine-containing slimicide.
It is characteristic of 4,5-dichloro-1,2-dithiol-3-one that it produces a rapid and potent microbicidal effect. While that is one of the most desirable characteristics of slimicies, 4,5-dichloro-1,2-dithiol-3-one itself is a relatively expensive compound and cost reduction has been demanded for its economical use. U.S. Pat. No. 3,871,860 discloses that bromonitrostyrene are useful for controlling slime-forming organisms in waters.